Don't get me wrong... it's part of OD&D, for better or worse. It reveals that it was very much a hobbyist effort at the outset. With no money for art, it's unsurprising that the original illustrations are what they are.

You can see the beginnings of a more professtional art approach, starting with Blackmoor (Greyhawk was still pretty bad) which really started to improve with the last couple of supplements. The only one I can't recall artwork for is the Swords & Spells supplement, which was supposed to replace Chainmail as the mass combat system